A systematic analysis of the structure, function, and regulation of the pyruvate and alpha-ketoglutarate dehydrogenase complexes from microorganisms and from mammalian tissues is continuing. Activity of the mammalian pyruvate dehydrogenase complex is regulated by phosphorylation and dephosphorylation catalyzed, respectively, by a MgATP2-dependent kinase and a Mg2 ion-dependent phosphatase. Phosphorylation occurs on three serine residues in the alpha-subunit of the pyruvate dehydrogenase component of the complex. Pyruvate dehydrogenase kinase and pyruvate dehydrogenase phosphatase have been isolated in a homogeneous state from bovine kidney and heart mitochondria. The subunit structure and regulatory properties of the kinase and the phosphatase are under investigation. The pyryvate and alpha-ketoglutarate dehydrogenase complexes are organized about a core consisting of dihydrolipoyl transacetylase or dihydrolipoyl transsuccinylase to which two other catalytic components, pyruvate dehydrogenase or alpha-ketoglutarate dehydrogenase and dihydrolipoyl dehydrogenase are joined by noncovalent bonds. Limited proteolysis of the dihydrolipoyl transacylases is being investigated to gain further insight into their subunit structure.